Chemistry problem help?

[John A.]

A student weighs an empty flask and stopper and finds the mass to be 54.868 g. She then adds about 5 mL of an unknown liquid and heats the flask in a boiling water bath at 100 C. After all the liquid is vaporized, she removes the flask from the bath, stoppers it, and lets it cool. After it is cool, she momentarily removes the stopper, then replaces it and weighs the flask and condensed vapor, obtaining a mass of 55.496 g. The volume of the flask is known to be 235.7 mL. The barometric pressure in the laboratory that day is 738 mm Hg.

A. What was the pressure of the vapor in the flask in atm?

B. How many moles of vapor are present?

C. What is the mass of one mole of vapor?

Thanks a lot